With respect to a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder for molding, hitherto there were some processes. For example, JP-B-44-22619 discloses a process for granulating by stirring in water in the presence of an organic liquid, and JP-B-44-22620 discloses a process for granulating by forming a PTFE powder into a slurry with the use of an organic liquid and then tumbling the slurry. However the processes disclosed in those patent publications have problems such that the organic liquid used is inflammable or harmful to human body and further a problem such that equipment cost for carrying out the processes is high.
On the contrary, with respect to a process without using an organic liquid, there is, for example, a process disclosed in JP-A-3-259925, in which a PTFE powder is stirred in water in the absence of an organic liquid. In such a process using water only, since an apparent density of an obtained PTFE granular powder for molding does not become high, post-treatment becomes necessary.
With respect to other process without using an organic liquid, there is, for example, a process disclosed in JP-B-54-17782, in which a PTFE powder is wetted with a specified amount of an aqueous solution containing a volatile nonionic surfactant and then subjected to tumbling. In the process disclosed in that patent publication, a granular powder having a low apparent density and excellent flowability cannot be obtained. Also since much amount of a nonionic surfactant is used and removal of it is difficult, it remains in a molded article, which results in causing problems such that mechanical properties of the molded article are lowered and that the surfactant is decomposed due to heat in a molding step, thereby causing coloring of the molded article.
Further, a process employing an ionic surfactant instead of the above-mentioned nonionic surfactant can be considered. However when a PTFE granular powder obtained by such a process is molded, a residue (for example, a metal salt) of the decomposed ionic surfactant is generated in the sintering step. The residue is difficult to remove, and has been considered to have problems of causing coloring of the molded article and lowering of mechanical properties of the molded article.
Also the PTFE powder is easy to be electrostatically charged, and thus when mixing, stirring and tumbling steps are carried out for granulation, there occurs electrostatic charge of not less than 50 V due to static electricity. Thus electrostatically charged PTFE powder, when molded, adheres to not only a mold die but also a hopper, feeder, etc. due to static electricity, which results in lowering of flowability.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that when granulating a PTFE powder and a filler by applying thereto a mechanical force such as tumbling, use of an anionic surfactant in a specified amount can solve the above-mentioned problems.
Namely an object of the present invention is to provide a process for preparing, without necessity of using an organic liquid, a filler-containing PTFE granular powder for molding which has a high apparent density, a small electrostatic charge and excellent powder flowability and gives a molded article which is free from lowering of mechanical properties such as elongation and coloring as mentioned above.